In the search for high performance materials, considerable interest has been focused upon carbon fibers. The terms "carbon" fibers or "carbonaceous" fibers are used herein in the generic sense and include graphite fibers as well as amorphous carbon fibers. Graphite fibers are defined herein as fibers which consist essentially of carbon and have a predominant x-ray diffraction pattern characteristic of graphite. Amorphous carbon fibers, on the other hand, are defined as fibers in which the bulk of the fiber weight can be attributed to carbon and which exhibit an essentially amorphous x-ray diffraction pattern. Graphite fibers generally have a higher Young's modulus than do amorphous carbon fibers and in addition are more highly electrically and thermally conductive.
Industrial high performance materials of the future are projected to make substantial utilization of fiber reinforced composites, and graphitic carbon fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density, high tensile strength, and high modulus.
As is known in the art, numerous procedures have been proposed in the past for the conversion of various organic polymeric fibrous materials to a carbonaceous form whiile retaining the orginal fibrous configuration essentially intact. Such procedures have in common the thermal treatment of the fibrous precursor in an appropriate atmosphere or atmospheres which is commonly conducted in a plurality of heating zones, or alternatively in a single heating zone wherein the fibrous material is subjected to progressively increasing temperatures. Both batch and continuous processing techniques have been proposed. From the commercial standpoint those processes which are capable of functioning on a continuous basis are generally considered to be the most attractive. However, many of the prior art continuous conversion techniques have been inherently limited to the processing of a single end of fibrous precursor at a given time. Such techniques while offering the advantages of possible automation, still suffer the disadvantage of limited productivity.
Techniques have heretofore been proposed for the simultaneous conversion of a substantial number of fibrous ends to a carbonaceous form which have involved the thermal treatment of a fibrous precursor while in the form of a woven cloth. See. for instance, Belgian Pat. Nos. 720,947 and 726,761, as well as U.S. Pat. No. 3,541,582 for representative disclosures of the processing of woven cloth precursors. However, the fiber bundles present in the conventionally woven carbon cloths commonly possess at least some permanent crimp at the warp and weft cross-over points and the single filament tensile properties of the fibers present within the cloths have tended to be adversely influenced.
There has arisen in the advanced engineering composite art the need for an efficient technique to produce pervious carbon fiber reinforced high strength composite articles of extremely low density. Woven carbon fabrics or cloths wherein weaving of a fibrous precursor was conducted prior to thermal conversion have been unsuitable for use in such applications because of (a) the high fiber density within the same and (b) impaired tensile properties resulting from fiber crimp. Prior attempts at the production of pervious low density carbon fiber reinforced composites have involved the tedious weaving of previously carbonized fiber bundles to form a substantially balanced cloth of an open weave construction which is subsequently resin impregnated with a matrix material. Such weaving by necessity must be conducted at a relatively slow rate because of the fragile nature of the previously carbonized fiber bundles. Even if such special weaving techniques are utilized, difficulties have arisen, however, with respect to the quality of fibrous reinforcement since the carbonized fiber bundles tend to be readily damaged during weaving with a concomitant diminution of their tensile properties.
It is an object of the invention to provide an improved process for the formation of pervious low density carbon fiber reinforced composite articles.
It is an object of the invention to provide a novel low density carbon fiber reinforced composite article comprising at least one layer or ply of a highly directional woven carbon fiber tape having an improved open weave construction which is impregnated with a substantially cured thermosetting resinous material.
It is an object of the invention to provide an improved process for forming a woven carbon fiber tape possessing an open weave construction suitable for use as a fibrous reinforcing medium in a pervious low density composite article.
It is an object of the invention to provide a novel carbon fiber tape of a highly directional open weave construction which is suitable for use as a fibrous reinforcing medium in a pervious low density composite article.
It is another object of the invention to provide improved pervious low density carbon fiber reinforced composite articles exhibiting superior translation of fiber properties into composite properties.
It is another object of the invention to provide improved pervious low density carbon fiber reinforced composite articles exhibiting a bulk density of about 0.4 to 1.4 grams/c.c.
It is a further object of the invention to provide pervious low density carbon fiber reinforced composite articles which exhibit excellent mechanical properties, and which are particularly suitable for use as facing sheets of an acoustic sandwich liner which serves as a noise suppression function in a turbofan nacelle for a jet engine.